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Isotope Geology Claude J. (cambridge)

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The color plates are situated between pages 220 and 221.
viii Contents
PREFACE
Isotopegeology is theo¡springofgeologyononehandandof the concepts andmethodsof
nuclearphysicsontheother. Itwas initiallyknownas‘‘nucleargeology’’andthenas‘‘isotope
geochemistry’’before its currentnameof isotopegeologycame tobepreferredbecause it is
basedonthemeasurementand interpretationofthe isotopic compositionsofchemical ele-
ments making up the various natural systems.Variations in these isotope compositions
yield useful information for the geological sciences (in the broad sense). The ¢rst break-
through for isotope geology was the age determination of rocks and minerals, which at a
stroke transformedgeology intoaquantitative science.Nextcamethemeasurementofpast
temperatures and the birth of paleoclimatology.Then horizons broadened with the emer-
genceofthe conceptof isotopictracerstoencompassnotonlyquestionsoftheEarth’s struc-
tures and internal dynamics, oferosion, andofthe transportofmaterial, but alsoproblems
ofcosmochemistry, including those relating totheoriginsofthe chemical elements.Andso
isotopegeologyhasnotonlyextendedacross the entire domainofthe earthsciencesbuthas
also expandedthatdomain, openingupmanynewareas, fromastrophysics to environmen-
tal studies.
This book is designed to provide an introduction to the methods, techniques, andmain
¢ndingsof isotopegeology.Thegeneral characterofthe subject de¢nes its potential reader-
ship: ¢nal-year undergraduates andpostgraduates in the earth sciences (or environmental
sciences), geologists, geophysicists, orclimatologistswantinganoverviewofthe¢eld.
This is an educational textbook.To my mind, an educational textbook must set out its
subject matter and explain it, but it must also involve readers in the various stages in the
reasoning. One cannot understand the development and the spirit of a science passively.
The reader must be active.This book therefore makes constant use of questions, exercises,
and problems. I have sought towrite a bookon isotope geology in the vein of Turcotte and
Schubert’s Geodynamics (Cambridge University Press) or Arthur Beiser’s Concepts of
ModernPhysics (McGraw-Hill),whichtomymindare exemplary.
As it is an educational textbook, information is sometimes repeated in di¡erentplaces.As
modern research in the neurosciences shows, learning is based on repetition, and so I have
adopted this approach. This is why, for example, although numerical constants are often
given in themain text,manyofthemare listedagain in tables at the end. Inother cases, I have
deliberatelynotgivenvalues sothat readerswill have to lookthemup for themselves, because
informationonehastoseekoutisrememberedbetter than informationserveduponaplate.
Readers must therefore work through the exercises, failing which they may not fully
understandhowthe ideas followon fromoneanother. Ihavegiven solutions aswegoalong,
sometimes in detail, sometimes more summarily. At the end of each chapter, I have set a
numberofproblemswhosesolutions canbefoundatthe endofthebook.
Another message Iwant to get across to students of isotope geology is that this is not an
isolated discipline. It is immersed both in the physical sciences and in the earth sciences.
Hence the deliberate use here and there of concepts from physics, from chemistry
(Boltzmanndistribution,Arrheniusequation,etc.),or fromgeology(platetectonics,petro-
graphy, etc.) to encourage study of these essential disciplines and, where need be, to make
readers look up information in basic textbooks. Isotope geology is the outcome of an
encounter between nuclear physics and geology; this multidisciplinary outlook must be
maintained.
Thisbookdoesnotsetouttoreviewall theresultsof isotopegeologybuttobringreadersto
apointwhere theycan consult theoriginal literature directlyandwithoutdi⁄culty.Among
current literature on the same topics, this book could be placed in the same category as
Gunter Faure’s IsotopeGeology (Wiley), to be read in preparation for AlanDickin’s excel-
lentRadiogenic IsotopeGeology (CambridgeUniversityPress).
The guideline I have opted to followhas been to leave aside axiomatic exposition and to
take instead a didactic, stepwise approach.The ¢nal chapter alone takes a more synthetic
perspective,whilegivingpointers for futuredevelopments.
I have to give a warning about the references. Since this is a book primarily directed
towards teaching I have notgiven a full setof references for each topic. I have endeavored to
give due credit to the signi¢cant contributorswith theproperorderofpriority (which is not
always the case in modern scienti¢c journals). Because it iswhat I ammost familiar with, I
havemade extensive use ofworkdone in mylaboratory.This leads to excessive emphasis on
myown laboratory’s contributions in some chapters. I feel sure my colleagues will forgive
meforthis.Thereferencesattheendofeachchapteraresupplementedbyalistofsuggestions
for further readingatthe endofthebook.
x Preface
ACKNOWLEDGMENTS
Iwould liketothankall thosewhohavehelpedme inwriting thisbook.
MycolleaguesBernardDupre¤ ,BrunoHamelin,E¤ ricLewin,Ge¤ rardManhe' s, andLaure
Meynadier made many suggestions and remarks right from the outset. Didier Bourles,
Serge Fourcade, Claude Jaupart, and Manuel Moreira actively reread parts of the
manuscript.
I am grateful too to thosewhohelped in producing the book: Sandra Jeunet, whoword-
processed a di⁄cult manuscript, Les E¤ ditions Belin, and above all Joe« l Dyon, who did the
graphics.ChristopherSutcli¡ehasbeenamostco-operativetranslator.
Myverysincerethanks toall.
CHAPTER ONE
Isotopes and radioactivity
1.1 Reminders about the atomic nucleus
In themodel¢rstdevelopedbyNiels BohrandErnestRutherford and extendedbyArnold
Sommerfeld, the atom is composed of two entities: a central nucleus, containing most of
themass, and an arrayoforbiting electrons.1The nucleus carries a positive charge ofþZe,
which is balanced by the electron cloud’s negative charge of�Ze.The number of protons,
Z, ismatched inan electricallyneutral atomby thenumberofelectrons.Eachoftheseparti-
cles carries anegative electric chargee.
As a rough description, the nucleus of any element is made up of two types of particle,
neutrons and protons. A neutron is slightly heavier than a proton with a mass of
mn¼ 1.674 95 � 10�27 kg compared with mp¼ 1.672 65 � 10�27 kg for the proton.While of
similar masses, then, the two particles di¡er above all in their charges. The proton has a
positive charge (þ e) while the neutron is electrically neutral.The numberofprotons (Z) is
the atomic number.The sumA¼NþZ of the numberof neutrons (N) plus the numberof
protons (Z) gives themass number.This provides a measure of the mass of the nuclide in
question if we take as our unit the approximate mass of the neutron or proton.Thomson
(1914)andAston (1919)showedthat, foragivenatomicnumberZ, thatis, foragivenposition
inMendeleyev’s periodic table, there are atomswith di¡erentmass numbersA, and there-
fore nuclei which di¡er in the numberof neutrons they contain (see Plate1). Such nuclides
areknownas the isotopesofan element.
Thus there isoneformofhydrogenwhosenucleus is composedof justa singleprotonand
another form of hydrogen (deuterium) whose nucleus comprises both a proton and a neu-
tron; these are the two stable isotopes of hydrogen. Most elements have several naturally
occurring isotopes. However, some, including sodium (Na), aluminum (Al), manganese
(Mn), andniobium(Nb),have justonenatural, stable isotope.
Theexistenceof isotopeshasgivenrisetoaspecial formofnotation for nuclides.Thesym-
boloftheelement ^ H,He,Li,etc.^ iscompletedbytheatomicnumberandthemassnumber
^ 11H;
2
1H;
6
3Li;
7
3Li, etc.Thisnotation leaves the right-handsideofthe symbol free forchemi-
cal notations used for molecularor crystalline compounds such as 21H2
16
8O2:The notation
atthe lowerleftcanbeomittedas